1. Field of the Invention
The invention relates to a medical implant which is suitable for implantation in an artificial bone bed on the mastoid region of the skull having a hermetically sealed housing in which an electronic component and optionally, other components or modules are accommodated.
2. Description of the Related Art
Implants of this general type are known in the art as exemplified by U.S. Pat. No. 5,411,467 to Hortmann et al. and by the published German patent application No. DE 39 18 329 A1. Such implants are surgically imbedded in the bony area of the skull behind the ear which is known as the mastoid. An artificial bone bed can be created in the mastoid to receive the housing of the implant. The designs of these implant housings are difficult in that there are severe size limitations. One major design limitation lies in the fact that the size of the bone bed must be kept as small as possible. Another limitation lies in the fact that the depth of the bone bed must be kept as shallow as possible. On the other hand, any projection of the housing above the outside edge of the bone bed would result in bulging of the skin above the housing which would not be desirable since one of the goals and benefits of such implants is to make the implants and the aiding devices inconspicuous. Of course, this design difficulty is exacerbated when the medical implant requires a volumetrically large housing. This can arise when the implant's electronics or other components are relatively large and complex such as a power supply unit or components thereof
In known implantable cochlea implants such as Nucleus 22 and Nucleus 24 Cochlea Implant System from Firma Cochlear AG, a receiver/stimulator electronic module is accommodated in a titanium housing which includes a silastic jacket. The silastic jacket forms a thin, flexible silastic flap (for example, 2.5 mm thick) which extends away from one side of the titanium housing and holds a receiving antenna coil together with a magnet. During the implantation, only the titanium housing is inserted into the bone bed while the thin flexible flap holding the antenna coil and/or magnet is placed on the outside of the bone bed. This flexible flap is then covered by the skin. The flexible silastic flap however, does not provide a hermetically air-tight sealing of the components enclosed therein. Although flexibility of the flap allows it to conform to the curved shape of the skull, there is a danger of breakage in the connections between the components held in the silastic flap and the components located in the titanium housing. Moreover, because the thickness of the flap is limited so that the skin over the flap does not disruptively bulge, this also severely limits the size of the implant components and the type of components which can be held within the flexible silastic flap.